Plates which are presently used to achieve osteosynthesis have a constant thickness due to the adopted production process. Said process consists in cutting, usually by laser, the plate from a sheet of biocompatible metal characterised by a constant thickness, following the plate contour, and in making the countersink holes for the screws heads by plastic deformation of the metal around the holes.
Osteosynthesis plates with constant thickness should have a thickness dimensioned in order to bear the tension in the most stressed section near the fracture or osteotomy line (proximal area). This thickness is however greater than the thickness required by the stress present in the sections farther from proximal area (distal areas). In these areas the adjustment of the plate to the bone surface done by bending can therefore be more difficult than needed. Another consequence of the constant thickness is that in the distal areas the elastic component of the deformation adopted to fit the plate to the bone geometry is greater than that resulting from a smaller thickness; unwanted stresses and displacements of the bone segments connected by the plate may be experienced.
In general the complex shape, the small size, the limited thickness in the proximal and distal areas of the plate make difficult the manufacturing of plates by metal cutting processes which would be necessary to obtain varying thickness, unless a system for fixing adequately the machined plate is available.
Fixing systems of magnetic type do not work with biocompatible materials such as titanium and stainless steel generally used for osteosynthesis plates. Other clamping systems of mechanical type in which the fastening is carried out by pressure on the workpiece do not permit to machine the plate without interruptions for changing the clamping zone and completing the operation in the plate areas previously hidden by the clamping device. The mechanical fastening doesn't prevent deformation and fracture in the plate areas with a small thickness since cutting loads can create stress conditions which cannot be allowed by the strength of the plate material. Moreover mechanical clamping may leave marks on the surface of the ductile materials used for the production of plates.
Fixing the workpiece by embedding in resin has the disadvantage of a poor dissipation of the heat produced during the workpiece cutting due to the inadequate thermal conductivity of the resin. Consequently the resin temperature increases determining the softening of the resin and the reduction of the resin binding capability necessary to keep the workpiece during the cutting process.
These drawbacks impede the industrial manufacturing by means of metal cutting processes, of osteosynthesis plates with a varying thickness and therefore prevent the achievement of the biomechanical advantages coming from the variation of the plate thickness.